The present application relates to the field of radiography examinations and imaging. It finds particular application with the calibration of direct conversion detector arrays and/or flat panel detectors, such as those commonly comprised within a computed tomography (CT) scanner, a line scanner, or other radiography imaging system (e.g., mammography system, general radiology system, etc).
Radiography imaging systems are useful to provide information, or images, of interior aspects of an object under examination. Generally, the object is exposed to radiation, and a two-dimensional image and/or a three-dimensional image is formed based upon the radiation absorbed by the interior aspects of the object, or rather an amount of radiation that is able to pass through the object. Typically, highly dense aspects of the object absorb more radiation than less dense aspects, and thus an aspect having a higher density, such as a bone, tumor or gun, for example, will be apparent when surrounded by less dense aspects, such as fatty tissue, muscle or clothing, for example.
A radiography imaging system typically comprises a detector array and a radiation source. The radiation source is generally configured to emit a fan, cone, wedge, or other shaped beam of radiation onto an object under examination. The detector array is generally positioned on a diametrically opposing side of the object relative to the radiation source and comprises a plurality of pixels that detect radiation that impinges upon the respective pixels. Typically, the pixels are configured to substantially continuously output an analog or digital signal, and when a charge density (e.g., proportional to the detected radiation) is measured by a pixel, the pixel is configured to emit a pulse, or change, in the respective signal indicative of the charge density. The signals emitted by the respective pixels can be converted into the digital domain (if not already in the digital domain) and used to generate an image(s) of the object showing areas of high radiation traversal and/or areas of low radiation traversal.
Periodically radiography imaging systems are calibrated to correct for gain, offset, defect correction, etc. caused by detector arrays or electronic equipment (e.g., amplifiers, readout devices, etc.) of the imaging systems. Generally, calibrations are performed at the factory before the detector array is attached to other portions of the imaging system. Calibrations are also typically performed during an air scan or flat field scan (e.g., a scan in which no object is present and/or an object with known characteristics is present). During a calibration, the detector array emits pulses indicative of radiation (e.g., x-rays) that is detected. Because no object is present or an object with predetermined/known characteristics is present, the pulses generated by the respective pixels are expected to exhibit predetermined characteristics and may be compared to such predetermined characteristics. Discrepancies between actual characteristics of a pulse and predetermined characteristics of the pulse may be attributed to an error (e.g., an undesirable gain, defect, etc.), and a correction factor that is configured to correct for the error may be identified. The correction factor may be applied to pulses generated by the pixel during an examination of an object to improve the quality of resulting images, for example.
While the aforementioned calibration techniques have proven effective for reducing errors and improving the quality of resulting images, there are several limitations/disadvantages to the calibration techniques. For example, the factory calibrations are time consuming because a high number of radiation exposures (e.g., 60 exposures or more) are generally required to reduce the impact of photon noise, for example. As a result, such a calibration can last two hours or more. Moreover, because the calibration techniques require the emission of radiation, safety restrictions are typically imposed. For example, a technician is generally required to monitor the calibration. Thus, a technician's time is consumed, and the radiation system is unavailable during a time when the system would typically be in operation.